Reactivation of herpes simplex virus type 1 (HSV-1) from a latent state in sensory neurons and anterograde transport to the cornea appears to be a predominant cause of recurrent herpes keratitis, a blinding disease of the cornea. Recent evidence from our laboratory and others strongly support an immunosurveillance role for CD8+ T cells in maintaining HSV-1 in a latent state in sensory neurons, and demonstrate that IFN- is employed to block reactivation in some, but not all neurons. Less well understood are the factors within latently infected tissue that influence the memory response. The goals of this proposal are two-fold: Specific Aim 1 will identify and characterize the effector mechanism used by CD8+ T cells to inhibit HSV-1 reactivation in neurons that are refractory to IFN-; and Specific Aim 2 will identify and characterize the factors responsible for maintaining the HSV-specific memory CD8+ population in the latently infected TG. In Specific Aim 1 we will confirm preliminary data strongly implicating CD8+ T cell lytic granules and specifically the component granzyme B (GrB) in controlling HSV-1 latency, and test the hypothesis that the use of lytic granules does not lead to neuronal destruction. We will compare the capacity of HSV-specific CD8+ T cells from wild type, perforin deficient (Pfn-/-) and GrB-/- mice to block HSV-1 reactivation in vivo and in ex vivo TG cultures in the presence and absence of GrB and caspase inhibitors; and use real time live cell fluorescence imaging to directly determine if CD8+ T cells release lytic granules into neurons, and if this interaction leads to activation of the caspase system and apoptosis in neurons. We will also test the hypothesis that the HSV-1 regulatory protein ICP4, via predicted GrB cleavage sites, competes with caspases as GrB substrate and is inactivated by cleavage resulting in GrB inhibition of HSV-1 reactivation without neuronal destruction. In Specific Aim 2 we will generate bone marrow chimera mice containing wild type CD8+ T cells and those deficient in receptors for cytokines responsible for homeostatic proliferation and survival of memory CD8+ T cells, and infect them with recombinant virus lacking the immunodominant epitope gB498-505 to compare the requirements for maintaining an HSV-specific CD8+ memory population in latently infected TG that do or do not express cognate antigen. We will also test the effect of latent virus on the requirement for CD4+ T cell help in generating functional CD8+ T cell memory, and determine if maintaining a CD8+ memory population in the TG requires replenishment from the peripheral blood. These studies will define the effect of a tissue microenvironment on maintenance of CD8+ T cell memory, an important factor that is often overlooked in vaccine design. Recurrent bouts of herpes simplex virus corneal disease result in progressive corneal scarring and represent an important cause of blindness world-wide. The recurrent nature of the disease is due to the ability of the virus to establish a latent (quiescent) infection in sensory neurons, and then periodically reactivate and infect the tissue innervated by those neurons. The goal of this grant is to provide an understanding of the mechanisms used by the host immune system to prevent the virus from reactivating from the latent state, as this information will be required to optimize the host immune response necessary to prevent recurrent herpetic disease.